Ethernet forwarding depends on a loop free active topology since looped frames cause network errors especially if the looped frames are multiplied during transmission such as in multicast and broadcast frames. The active topology is configured by Rapid Spanning Tree Protocol (RSTP) or Multiple Spanning Tree Protocol (MSTP) in current Ethernet networks. Extensions are being defined for Ethernet as it is becoming more popular in network segments (besides LANs) such as in metro core and backbone networks. As a result of the increased popularity, a need exists for defining mechanisms (other than MSTP) for configuring the active topology.
There are currently two tracks under development within IEEE 802.1aq that are aimed at replacing MSTP with Intermediate System to Intermediate System (IS-IS) link-state protocol.
In one track, IS-IS is used for the control of the bridge network where traditional MAC learning is applied (i.e. a bridge learns the reachability of a MAC address if it sees an Ethernet frame sent from that MAC address that may be applied in an enterprise or in a campus network where MAC addresses are unmanaged for example). This track is referred to as Shortest Path Bridging (SPB).
In the other track, IS-IS is applied in a Provider Backbone Bridge (PBB) Network where all addresses are managed and unknown addresses do not appear in the network. In such an environment, no spanning tree protocol is applied but Filtering Databases (FDBs) are set up and updated directly by the IS-IS control protocol. This track is referred to as Shortest Path Backbone Bridging (SPBB).
Accidental loops may appear due to the unsynchronized update of FDBs by a link-state control protocol such as IS-IS during a topology transient (i.e. changing topology). While ingress filtering mechanism can be applied to eliminate loop of unicast traffic, it is not suitable for eliminating all possible loops of multicast traffic. What is desired, therefore, is a mechanism for preventing loops in multicast traffic.
The Mutiple Registration Protocol (MRP) under IEEE 802.1 ak enables participants (i.e. network nodes intending to participate in a specific service such as in a VLAN or in a multicast tree) in a MRP application to register attributes with other participants in a Bridged Local Area Network (BLAN). This means that an MRP application is defined for a specific service and bridges and bridge ports that aim to participate in that specific service are able to register through this MRP application.
Two Applications are defined for registering virtual LANs (VLANs) and group MAC addresses. These applications are: (1) Multiple VLAN Registration Protocol (MVRP) where bridges can register to a VLAN and the aim is to provide automatic configuration of VLANs instead of manual configuration; (2) Multiple MAC registration protocol (MMRP) which provides automatic registration of MAC addresses to a multicast service. MRP is a standard protocol for controlling the forwarding over the active topology.
IEEE does not provide an alternative mechanism to MSTP for the configuration of the active topologies (i.e. for setting up and updating spanning trees). IS-IS is proposed as a control protocol for the configuration of spanning trees within SPB. However, the loop prevention mechanisms under discussion have significant drawbacks. Reverse Forwarding Path Check, which is an ingress checking mechanism, is proposed for SPBB. Accidental loops may appear during a topology transient from where multicast frames may propagate to the rest of the network (also referred to as Catherine-wheel) which causes multiple reception of the same or identical frames at the destination thus degrading the quality in a real-time application.
While some sequences are proposed for setting up loop free active topologies, the protocols applied there do not include MRP which is a suitable IEEE standard in order to set up trees.